Yeah, it's a tricky one -- if people ASK, then it's okay to suggest something like a gift certificate to the local zoo, or to a kids play place, or some other thing that you can use up. Otherwise, just try to keep things in their original packaging, and return as much as you can. A sneaky way to find out where someone got a gift is to (upon opening it) say, "Wow -- look at that, (son's name)! Now (giver's name), where on EARTH did you find such a clever gift?" Hopefully they'll disclose the store, and you can then discreetly return it.

(Obviously you can't use this line for every gift, so just pick the gifts that, upon opening them, give you an inner mental reaction of "Oh, HELL no!")

In my experience, it's really hard to do that. We keep everything in the packaging and then either regift or donate or sell to a consignment shop (sometimes we even get credit at the consignment shop and can use it for things we actually need, like shoes!)